Very useful to determine if a mod should be doing something if "unit" is dead.
Returns 1 if "unit" is dead, nil otherwise.
Information from UnitIsDeadOrGhost("unit"):
Does not work for ("pet") unless the pet bar is still active. Not sure why the petbar stays active when the pet dies -- it used to gray out, now it will disappear after some time or some event happens, but not immediately when the pet dies. -- 5/29/2006